Enable Pipeline Parallelism to use mp as distributed backend on Jax TPU platform (vllm-project#1054)

Signed-off-by: Chenyaaang <chenyangli@google.com>

Author: Chenyaaang

tpu_inference/platforms/tpu_platform.py

@@ -184,8 +184,14 @@ def check_and_update_config(cls, vllm_config: VllmConfig) -> None:
 
         multihost_backend = os.environ.get("TPU_MULTIHOST_BACKEND", "").lower()
         if not multihost_backend:  # Single host
-            logger.info("Force using UniProcExecutor for JAX on single host.")
-            parallel_config.distributed_executor_backend = "uni"
+            if parallel_config.pipeline_parallel_size == 1:
+                logger.info("Force using UniProcExecutor for JAX on \
+                        single host without pipeline parallelism.")
+                parallel_config.distributed_executor_backend = "uni"
+            else:
+                logger.info("Force using MultiprocExecutor for JAX on \
+                        single host with pipeline parallelism.")
+                parallel_config.distributed_executor_backend = "mp"
         elif multihost_backend == "ray":
             from tpu_inference.executors.ray_distributed_executor import \
                 RayDistributedExecutor